Configurational stereoisomer of difethialone, composition and rodenticidal bait comprising same, and process for controlling target rodent pests

ABSTRACT

Disclosed is a dextrorotatory enantiomer of the configurational stereoisomer of difethialone, named homo-stereoisomer, the formula of which is 3-(4′-bromobiphenyl-4-yl)-1-(4-hydroxythiocoumarin-3-yl)-1,2,3,4-tetrahydronaphthalene, in which carbons 1 and 3 of the 1,2,3,4-tetrahydronaphthalene group of the homo-stereoisomer have the same absolute configuration.

FIELD OF THE INVENTION

The invention relates to a configurational stereoisomer of difethialone as an isolated compound, to a composition and a rodenticidal bait comprising such a configurational stereoisomer and to a process for controlling target rodent pests. The invention thus relates to the technical field of controlling populations of target rodent pests.

BACKGROUND OF THE INVENTION

It is known practice to use poisons in the form of rodenticidal baits against target rodent pests. It is known from EP 2 090 164 that difethialone is a second-generation anticoagulant acting in a single dose manner. US 2005/181003 describes a rodenticidal bait in gel form comprising difethialone in a mass proportion of 25 ppm.

Such a bait is liable to be consumed by animals other than target rodent pests when it is made available to target rodent pests. It may be consumed directly (primary consumption) by domestic animals or pets. It may also be consumed accidentally by humans. Such consumption may result in poisoning, which may be lethal, of these domestic animals, pets or humans.

In addition, a fraction of the difethialone of these rodenticidal baits may be ingested (secondary consumption) by animals—especially by birds—which prey on weakened rodent pests that have consumed such a rodenticidal bait, or by animals which carrion-feed on rodent pests that have died from having consumed such a rodenticidal bait. This secondary consumption is liable in the long term to result in the death of these predatory or carrion-feeding animals, which may be animals—especially birds—belonging to protected species.

SUMMARY OF THE INVENTION

The invention is thus directed towards overcoming these drawbacks by proposing a configurational stereoisomer of difethialone, a composition and a rodenticidal bait comprising such a configurational stereoisomer and a process for controlling harmful rodent pests, which are not only effective for controlling the populations of target rodent pests but can also limit the risks of poisoning of non-target animals—especially domestic or reared animals, pets or humans—which accidentally consume such a rodenticidal bait.

The invention is thus directed towards overcoming these drawbacks by proposing a configurational stereoisomer of difethialone, a composition and a rodenticidal bait comprising such a configurational stereoisomer and a process for controlling target rodent pests, which are not only effective for controlling the populations of target rodent pests but can also limit the risks of secondary poisoning of wild animals—for example foxes or birds—which prey on weakened target rodent pests that have consumed the rodenticidal bait or of wild animals which carrion-feed on target rodent pests that died from being poisoned.

The invention is also directed towards proposing a configurational stereoisomer of difethialone, a composition and a rodenticidal bait comprising such a configurational stereoisomer and a process for controlling target rodent pests, the use of which is in accordance with the rules of good practice, especially with respect to the protection of birds, and in particular birds of prey.

The invention is also directed towards proposing a configurational stereoisomer of difethialone, a composition and a rodenticidal bait comprising such a configurational stereoisomer and a process for controlling target rodent pests, which do not require, in order to control a population of target rodent pests, the use of a rodenticidal agent at high dose and which are friendly towards the environment and the health of humans and non-target animals—especially birds.

The invention is also directed towards proposing a configurational stereoisomer of difethialone, a composition and a rodenticidal bait comprising such a configurational stereoisomer and a process for controlling target rodent pests, which are able to be used for controlling target rodent pests that are resistant to known baits for controlling target rodent pests.

The invention is thus directed towards proposing an alternative to known rodenticidal baits.

To do this, the invention relates to a dextrorotatory enantiomer of a configurational stereoisomer of difethialone, named homo-stereoisomer, the formula of which is 3-(4′-bromobiphenyl-4-yl)-1-(4-hydroxythiocoumarin-3-yl)-1,2,3,4-tetrahydronaphthalene, in which carbons 1 and 3 of the 1,2,3,4-tetrahydronaphthalene group of said homo-stereoisomer have the same absolute configuration.

Throughout the text:

-   -   the term “difethialone” denotes the compound         3-(4′-bromobiphenyl-4-yl)-1-(4-hydroxythiocoumarin-3-yl)-1,2,3,4-tetrahydronaphthalene         or         3-[3-[4-(4-bromophenyl)phenyl]-1-tetralinyl]-2-hydroxy-4-thiochromenone         or 3-[3-(4′-bromo         [1,1′-biphenyl]-4-yl)-1,2,3,4-tetrahydro-1-naphthalenyl]-4-hydroxy-2H-1-benzothiopyran-2-one         of formula (I) below:

in which the numbers of carbons 1 and 3 of the 1,2,3,4-tetrahydronaphthalene group are indicated;

-   -   the term “stereoisomers” denotes isomers of the same         semi-structural formula, but in which the relative position of         the atoms differs in space. The term “configurational         stereoisomers” denotes stereoisomers for which conversion from         one to the other of these configurational stereoisomers requires         the cleavage/reformation of an interatomic covalent bond. Thus,         the term “configurational stereoisomers” denotes stereoisomers         which are not conformational isomers (or “rotamers”, for which         conversion from one to the other of the conformational isomers         is accompanied only by rotation of a part of the molecule about         the axis of a σ (sigma) bond formed by axial orbital overlap);     -   the term “hetero-stereoisomer” of difethialone denotes the         configurational stereoisomer of difethialone of formula         3-(4′-bromobiphenyl-4-yl)-1-(4-hydroxythiocoumarin-3-yl)-1,2,3,4-tetrahydronaphthalene,         in which carbons 1 and 3 of the 1,2,3,4-tetrahydronaphthalene         group of said hetero-stereoisomer have different absolute         configurations (i.e. 1S-3R and 1R-3S), the absolute         configurations being determined according to the sequential         priority rules and the Cahn-Ingold-Prelog (CIP) nomenclature;     -   the term “homo-stereoisomer” of difethialone denotes the         configurational stereoisomer of difethialone of formula         3-(4′-bromobiphenyl-4-yl)-1-(4-hydroxythiocoumarin-3-yl)-1,2,3,4-tetrahydronaphthalene,         in which carbons 1 and 3 of the 1,2,3,4-tetrahydronaphthalene         group of said homo-stereoisomer have the same absolute         configuration (i.e. 1S-3S and 1R-3R);     -   the term “amount” means a molar amount, a mass amount or a         volume amount. The proportions are thus proportions of a molar         amount relative to a molar amount, of a mass amount relative to         a mass amount, or of a volume amount relative to a volume         amount;     -   the term “substantially” indicates, in the usual manner, that a         structural or functional characteristic should not be taken as         marking an abrupt discontinuity, which would have no physical         meaning, but covers not only this structure or this function,         but also slight variations of this structure or of this function         which produce, in the technical context under consideration, an         effect of the same nature, or else of the same degree;     -   the expressions “high-pressure liquid chromatography” or         “high-performance liquid chromatography” (HPLC) denote “HPLC”         chromatography or “High-Performance Liquid Chromatography”; and     -   the term “retention time” denotes the time, measured at the top         of the peak in the chromatogram, for which a compound is         retained on a chromatography column.

The invention relates to the dextrorotatory enantiomer of said homo-stereoisomer of difethialone. The invention relates to the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in isolated form and in particular separated from the laevorotatory enantiomer of said homo-stereoisomer of difethialone and from the laevorotatory and dextrorotatory enantiomers of a configurational stereoisomer of difethialone, named hetero-stereoisomer, in which carbons 1 and 3 of the 1,2,3,4-tetrahydronaphthalene group have different absolute configurations.

The inventors have discovered that it is possible to separate the laevorotatory and dextrorotatory enantiomers of said homo-stereoisomer of difethialone and the laevorotatory and dextrorotatory enantiomers of said hetero-stereoisomer of difethialone by high-pressure liquid chromatography in isocratic mode and under particular conditions by using a chromatography column comprising a chiral stationary phase. Specifically, it was not known at the date of the invention how to separate the configurational stereoisomers of difethialone and especially the laevorotatory enantiomer and the dextrorotatory enantiomer of said homo-stereoisomer of difethialone.

The inventors succeeded in performing this separation by choosing a particular HPLC chromatography column, LUX® Cellulose-3 (Phenomenex, Le Pecq, France) of dimensions 150×2 mm and comprising a chiral stationary phase constituted of porous particles of cellulose tris(4-methylbenzoate), having a particle size of 3 μm and a porosity of 1000 Å. They used, as mobile phase, an eluent formed from a mixture of acetonitrile (A) and water comprising formic acid in a volume proportion of 0.1% in water (B) with an A/B volume ratio of 80/20. The flow rate of the mobile phase in the column is maintained at a value of 0.25 mL/minute and the separation is performed at a temperature of 23.2° C. The composition to be analysed is at a concentration of 1 μg of difethialone per millilitre in acetonitrile and the volume injected onto the column is 1 μL. Detection may be performed by tandem mass spectrometry (MS/MS). Detection may also be performed by photometry or by spectrophotometry by adjusting the difethialone concentration and the injection volume for the purpose of obtaining optimum detection and by measuring the value of the area under the peak for each configurational stereoisomer.

Under these experimental conditions, the value of the retention time (t₄) for the dextrorotatory enantiomer of said homo-stereoisomer according to the invention may vary according to the operating conditions—especially according to the column temperature conditions—and may be between 14.0 minutes and 14.4 minutes. The value of the retention time (t₁) for the laevorotatory enantiomer of said homo-stereoisomer may vary according to the operating conditions—especially according to the column temperature conditions—and may be between 7.8 minutes and 8.2 minutes, such that the dextrorotatory and laevorotatory enantiomers of said homo-stereoisomer may be separated by high-pressure liquid chromatography on a chiral column.

Under these same experimental conditions, the value of the retention time (t₃) for the dextrorotatory enantiomer of said hetero-stereoisomer of difethialone may vary according to the operating conditions—especially according to the column temperature conditions—and may be between 11.3 minutes and 11.8 minutes. The value of the retention time (t₂) for the laevorotatory enantiomer of said hetero-stereoisomer of difethialone may vary according to the operating conditions—especially according to the column temperature conditions—and may be between 9.0 minutes and 9.5 minutes.

Thus, under these analytical conditions, the order of elution of the configurational stereoisomers of difethialone is such that t₁<t₂<t₃<t₄. The retention time values t₁, t₂, t₃ and t₄ are liable to vary, especially with the temperature of the chromatography column. However, under these chromatographic conditions, the order of elution of the configurational stereoisomers of difethialone remains unchanged.

The invention thus relates to the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in isolated form and having the property of being able to be eluted, under the chromatography conditions described above, last from among the four configurational stereoisomers of difethialone.

The dextrorotatory enantiomer of said homo-stereoisomer of difethialone isolated in pure form according to the invention, dissolved in methanol at a concentration of 0.96 g/L and placed in a quartz spectrophotometry cuvette, has a circular dichroism spectrum acquired at 25° C. with positive circular dichroism values between 220 nm and 300 nm.

The dextrorotatory enantiomer of said homo-stereoisomer of difethialone isolated in pure form according to the invention, dissolved in chloroform (CHCl₃) has a specific optical rotation [α]^(25° C.) _(589nm), measured at 25° C. and on the sodium D line (589 nm), having a value of +19.9°.

The dextrorotatory enantiomer of said homo-stereoisomer of difethialone isolated in pure form according to the invention has, on proton magnetic resonance (¹H-NMR) spectroscopy at 500 MHz in CDCl₃, a multiplet with a chemical shift (δ) of between 4.9 ppm and 5.1 ppm corresponding to the proton borne by carbon 1 of the 1,2,3,4-tetrahydronaphthalene group of said homo-stereoisomer of difethialone.

Said hetero-stereoisomer of difethialone and said homo-stereoisomer of difethialone are distinguished by their proton NMR spectra. In the proton NMR spectrum acquired in CDCl₃, the chemical shift of the proton borne by carbon 1 of the 1,2,3,4-tetrahydronaphthalene group of said hetero-stereoisomer of difethialone is about 5.3 ppm.

The invention also relates to a composition comprising the dextrorotatory enantiomer of said homo-stereoisomer of difethialone according to the invention, with the exclusion of a racemic mixture of dextrorotatory and laevorotatory enantiomers of said homo-stereoisomer of difethialone, i.e. of an optically inactive mixture of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone and of the laevorotatory enantiomer of said homo-stereoisomer of difethialone.

The invention thus also relates to a composition comprising a dextrorotatory enantiomer of the configurational stereoisomer of difethialone, named homo-stereoisomer, of formula 3-(4′-bromobiphenyl-4-yl)-1-(4-hydroxythiocoumarin-3-yl)-1,2,3,4-tetrahydronaphthalene, in which carbons 1 and 3 of the 1,2,3,4-tetrahydronaphthalene group of said homo-stereoisomer have the same absolute configuration, with the exclusion of a mixture in which the dextrorotatory enantiomer and the laevorotatory enantiomer of said homo-stereoisomer of difethialone are in the same amount.

Advantageously and according to the invention, said homo-stereoisomer is predominantly in dextrorotatory enantiomer form.

Throughout the text, the term “said homo-stereoisomer is predominantly in dextrorotatory enantiomer form” means that the (mass, molar or volume) amount of dextrorotatory enantiomer of said homo-stereoisomer of difethialone is the major amount—greater than 50%—in all of said homo-stereoisomer of difethialone present in the composition (in all its dextrorotatory and laevorotatory enantiomer forms), i.e. the (mass, molar or volume) amount of dextrorotatory enantiomer of said homo-stereoisomer of difethialone in the composition is such that the ratio of this amount to the (total) amount of said homo-stereoisomer in the composition is greater than 50%.

In particular, in a composition according to the invention:

-   -   the ratio of the amount of dextrorotatory enantiomer of said         homo-stereoisomer of difethialone to the sum of the amounts of         each of the (dextrorotatory and laevorotatory) enantiomers of         said homo-stereoisomer of difethialone is greater than 0.5         (greater than 50%);     -   the ratio of the concentration of dextrorotatory enantiomer of         said homo-stereoisomer of difethialone to the sum of the         concentrations of each of the (dextrorotatory and laevorotatory)         enantiomers of said homo-stereoisomer of difethialone is greater         than 0.5 (greater than 50%); and     -   the proportion of dextrorotatory enantiomer of said         homo-stereoisomer of difethialone in the composition is greater         than the proportion of the laevorotatory enantiomer of said         homo-stereoisomer of difethialone.

Advantageously and according to the invention, the composition comprises an amount of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in the composition such that the ratio of this amount to the total amount of said homo-stereoisomer of difethialone in the composition is greater than 50%, especially greater than 60%, in particular greater than 70%, more particularly greater than 80%, preferably greater than 90%, more preferentially greater than 95%, particularly preferentially greater than 98%, even more preferentially greater than 99% or about 100%. Advantageously, it comprises an amount of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in the composition such that the ratio of this amount to the total amount of said homo-stereoisomer of difethialone in the composition is between 85% and 100%, more preferentially between 90% and 98%. Advantageously, it comprises an amount of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in the composition such that the ratio of this amount to the total amount of said homo-stereoisomer of difethialone in the composition is between 98% and 100%.

Advantageously, the composition may also comprise the laevorotatory enantiomer of said homo-stereoisomer of difethialone, but in a proportion in the composition which is less than 50%—especially less than 25%, preferentially between 0% and 25%, in particular less than 10%—relative to the homo-stereoisomer of difethialone.

Advantageously and according to the invention, the difethialone is predominantly in the dextrorotatory enantiomer form of said homo-stereoisomer of difethialone. The composition comprises an amount of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in the composition such that the ratio of this amount to the amount of each of the enantiomers (of said hetero-stereoisomer and of said homo-stereoisomer) of difethialone is greater than 25%.

Advantageously, in a composition according to the invention:

-   -   the ratio of the amount of dextrorotatory enantiomer of said         homo-stereoisomer of difethialone to the sum of the amounts of         each of the enantiomers of said homo-stereoisomer of         difethialone and of each of the enantiomers of said         hetero-stereoisomer of difethialone is greater than 0.25         (greater than 25%);     -   the ratio of the concentration of dextrorotatory enantiomer of         said homo-stereoisomer of difethialone relative to the sum of         the concentrations of each of the enantiomers of said         homo-stereoisomer of difethialone and of each of the enantiomers         of said hetero-stereoisomer of difethialone is greater than 0.25         (greater than 25%); and     -   the proportion of dextrorotatory enantiomer of said         homo-stereoisomer of difethialone in the composition is greater         than the proportion of each of the enantiomers of said         homo-stereoisomer of difethialone and of said         hetero-stereoisomer of difethialone. In a composition according         to the invention, the proportion of dextrorotatory enantiomer of         said homo-stereoisomer of difethialone is greater than 25%         relative to the (total) difethialone.

Advantageously and according to the invention, the composition comprises an amount of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in the composition such that the ratio of this amount to the amount of total difethialone in the composition is greater than 25%, especially greater than 50%, in particular greater than 70%, more particularly greater than 80%, preferably greater than 90%, particularly preferentially greater than 95%, more preferentially greater than 98%, even more preferentially greater than 99% or about 100%. A composition according to the invention thus comprises difethialone predominantly in dextrorotatory enantiomer form of said homo-stereoisomer of difethialone.

Advantageously, the composition comprises an amount of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone such that the ratio of this amount to the amount of total difethialone is greater than 70%, preferably between 80% and 100%, more preferentially between 90% and 100%. Advantageously, the composition comprises an amount of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone such that the ratio of this amount to the amount of total difethialone is between 95% and 99%. Advantageously, it comprises an amount of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone such that the ratio of this amount to the amount of total difethialone is between 98% and 100%. Advantageously, the composition comprises an amount of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone such that the ratio of this amount to the amount of total difethialone is about 100%.

Advantageously and according to the invention, the composition comprises an amount of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in the composition such that the ratio of this amount to the amount of total difethialone in the composition is greater than 97%.

A composition according to the invention may be substantially free of laevorotatory enantiomer of said homo-stereoisomer of difethialone, i.e. the laevorotatory enantiomer of said homo-stereoisomer of difethialone may be present in the composition, but only in trace amount. It may also be substantially free of said hetero-stereoisomer of difethialone, i.e. said hetero-stereoisomer of difethialone may be present in the composition, but only in trace amount.

Advantageously and according to the invention, the composition is in liquid form and comprises a liquid solvent for difethialone. It may be a solution of difethialone in a solvent for difethialone, with the exclusion of a racemic mixture of said laevorotatory and dextrorotatory enantiomers of said homo-stereoisomer of difethialone. It may also be a solution comprising difethialone in a solvent for difethialone and in which said homo-stereoisomer of difethialone is predominantly in dextrorotatory enantiomer form. It may also be a solution comprising difethialone in a solvent for difethialone and in which the difethialone is predominantly in the dextrorotatory enantiomer form of said homo-stereoisomer of difethialone.

Advantageously and according to the invention, the composition is in solid form. It may also be a solid comprising difethialone, with the exclusion of a racemic mixture of the dextrorotatory and laevorotatory enantiomers of said homo-stereoisomer of difethialone. It may also be a solid comprising difethialone and in which said homo-stereoisomer of difethialone is predominantly in dextrorotatory enantiomer form. It may also be a solid comprising difethialone and in which the difethialone is predominantly in the dextrorotatory enantiomer form of said homo-stereoisomer of difethialone.

The invention thus also relates to a composition comprising the dextrorotatory enantiomer according to the invention, with the exclusion of a racemic mixture of the laevorotatory and dextrorotatory enantiomers of said homo-stereoisomer of difethialone, the difethialone of the composition being optically active and dextrorotatory. However, it is not excluded for the composition according to the invention, comprising the dextrorotatory enantiomer according to the invention with the exclusion of a racemic mixture of the laevorotatory and dextrorotatory enantiomers of said homo-stereoisomer of difethialone, to itself be optically active and laevorotatory, or alternatively optically inactive.

The invention also relates to the use of a composition according to the invention for the preparation of a rodenticidal bait for target rodent pests.

The invention also relates to a rodenticidal bait comprising a composition according to the invention, and at least one excipient that is edible for target rodent pests.

A rodenticidal bait according to the invention comprises:

-   -   at least one excipient that is edible for target rodent pests,     -   the dextrorotatory enantiomer of a configurational stereoisomer         of difethialone, named homo-stereoisomer, of formula         3-(4′-bromobiphenyl-4-yl)-1-(4-hydroxythiocoumarin-3-yl)-1,2,3,4-tetrahydronaphthalene,         in which carbons 1 and 3 of the 1,2,3,4-tetrahydronaphthalene         group of said homo-stereoisomer have the same absolute         configuration, with the exclusion of a racemic mixture of         dextrorotatory and laevorotatory enantiomers of said         homo-stereoisomer of difethialone. In a bait according to the         invention, the dextrorotatory enantiomer of said         homo-stereoisomer of difethialone and the laevorotatory         enantiomer of said homo-stereoisomer of difethialone are in         different amounts.

Advantageously, a bait according to the invention comprises an excipient that is edible for target rodent pests and said homo-stereoisomer of difethialone predominantly in dextrorotatory enantiomer form.

The inventors, who succeeded in separating the dextrorotatory enantiomer from the laevorotatory enantiomer of said homo-stereoisomer of difethialone and from said hetero-stereoisomer of difethialone and in isolating the dextrorotatory enantiomer of said homo-stereoisomer of difethialone, also observed, entirely surprisingly, that the dextrorotatory enantiomer of said homo-stereoisomer of difethialone and the laevorotatory enantiomer of said homo-stereoisomer of difethialone do not have the same persistence in the liver of target rodent pests and that the dextrorotatory enantiomer of said homo-stereoisomer of difethialone is in fact the enantiomer of said homo-stereoisomer of difethialone that is the less persistent in the liver of target rodents, especially of target rodent pests.

They observed that the dextrorotatory enantiomer of said homo-stereoisomer of difethialone, which is less persistent and disappears from the liver of the target rodent pest which has consumed a bait according to the invention more rapidly than the disappearance of the laevorotatory enantiomer of said homo-stereoisomer of difethialone, is, however, rodenticidal.

The residual dextrorotatory enantiomer of said homo-stereoisomer of difethialone in the body of a poisoned rodent disappears more quickly than the other (laevorotatory) enantiomer of said homo-stereoisomer of difethialone. The dead or live target rodent pest which has ingested the dextrorotatory enantiomer of said homo-stereoisomer of difethialone is thus sparingly toxic with respect to non-rodent mammals and birds which consume the target rodent pest—dead or alive—and in particular with respect to predators (especially non-rodent mammals and birds) which preferentially consume the viscera of their prey and in particular their liver.

The inventors have also observed that the dextrorotatory enantiomer of said homo-stereoisomer of difethialone, although having low hepatic persistence in target rodent pests, does in fact make it possible, in an entirely surprising manner to date, to efficiently control target rodent pests.

Advantageously and according to the invention, the rodenticidal bait comprises a mass amount of difethialone such that the ratio (mass proportion) of this mass amount of difethialone to the mass amount of rodenticidal bait is less than 200 ppm, i.e. less than 200 mg of difethialone per kilogram of bait. Advantageously, the mass proportion of difethialone in the rodenticidal bait is between 1 ppm and 100 ppm (1 mg to 100 mg of difethialone per kilogram of rodenticidal bait), especially between 5 ppm and 100 ppm (5 mg to 100 mg of difethialone per kilogram of rodenticidal bait), preferably between 5 ppm and 50 ppm (5 mg to 50 mg of difethialone per kilogram of rodenticidal bait), more preferentially between 10 ppm and 50 ppm (10 mg to 50 mg of difethialone per kilogram of rodenticidal bait), even more preferentially between 15 ppm and 50 ppm (15 mg to 50 mg of difethialone per kilogram of rodenticidal bait), for example about 15 ppm (15 mg of difethialone per kilogram of bait).

Advantageously and according to the invention, the edible excipient comprises at least one food chosen from the group formed from cereal seeds, especially hulled cereal seeds, cereal seed meals, cereal seed flours, cereal seed flakes, cereal bran and non-cereal seeds, for example alfalfa seeds, especially in hulled form, in the form of meal, in the form of flour, or in the form of flakes or bran. The edible excipient may comprise any support that can be consumed by target rodent pests.

Advantageously, the edible excipient comprises at least one food chosen from the group formed from foods of plant origin and foods of animal origin. Advantageously, the edible excipient comprises at least one food chosen to stimulate the appetite of the target rodent pests. In particular, this food is chosen from the group formed from seeds of one or more cereals, hulled seeds of one or more cereals, meals of seeds of one or more cereals, flakes of seeds of one or more cereals, bran of one or more cereals and flour of seeds of one or more cereals. By way of example, the cereals are chosen from the group formed from oat, wheat, barley, corn, soybean and rice.

Advantageously, the food is chosen from the group formed from sweetened foods. For example, they may be foods comprising at least one sugar chosen from the group formed from sucrose, lactose, fructose and glucose. It may be a sugar syrup—for example a sugar syrup obtained by hydrolysis of starch—or a sugar syrup obtained by hydrolysis of sucrose (invert sugar syrup), or a beet sugar syrup, or a maple syrup or a sugarcane syrup, or a syrup obtained from a plant of the Stevia genus.

Advantageously, the food is chosen from the group formed from coconut albumen (copra) flakes and flour. Advantageously, the food is chosen from the group formed from walnuts, hazelnuts and almonds—in grated and/or powder form.

Advantageously, the food is chosen from the group formed from plant fats, plant oils (for example rapeseed oil, soybean fat, sunflower oil, cocoa butter, groundnut oil, groundnut butter, corn oil, palm oil), animal fats and animal oils (butter, lard, fish oil).

Advantageously, the food is chosen from the group formed from proteins of plant origin and proteins of animal origin. By way of example, examples that may be mentioned include powdered milk, especially powdered skimmed milk, eggs, especially powdered eggs, protein hydrolysates of animal origin and protein hydrolysates of plant origin.

The excipient that is edible for target rodent pests is chosen to allow consumption of the bait by target rodent pests. Advantageously and according to the invention, each edible excipient is non-lethal to target rodent pests. The edible excipient is not in itself rodenticidal.

Advantageously and according to the invention, the rodenticidal bait is chosen from the group formed from solid baits comprising difethialone and a solid edible excipient. Advantageously, the rodenticidal bait is a solid in divided form, for example in the form of balls or granules. Advantageously, the rodenticidal bait may be a solid in block or paste form that may be consumed by the target rodent pests or a solid material that may be nibbled by the target rodent pests. Advantageously, the solid rodenticidal bait according to the invention may be in the form of a rigid block, a semi-rigid block, a foam, a powder or a gel.

Advantageously, the rodenticidal bait which is in the form of a powder, in the form of a foam or in the form of a gel is suitable for soiling the fur of the target rodent pest(s) and for being ingested by said pest(s) during their grooming.

It may be a solid rodenticidal bait comprising difethialone, with the exclusion of a racemic mixture of the dextrorotatory and laevorotatory enantiomers of said homo-stereoisomer of difethialone. It may also be a solid rodenticidal bait comprising difethialone and in which said homo-stereoisomer of difethialone is predominantly in dextrorotatory enantiomer form. It may also be a solid rodenticidal bait comprising difethialone and in which the difethialone is predominantly in the dextrorotatory enantiomer form of said homo-stereoisomer of difethialone.

Advantageously and according to the invention, the rodenticidal bait is chosen from the group formed from liquid baits comprising difethialone and a liquid edible excipient. The rodenticidal bait is then a drink for target rodent pests.

Advantageously and according to the invention, the rodenticidal bait is chosen from the group formed from liquid baits comprising difethialone and a liquid edible excipient. The rodenticidal bait is then a drink for target rodent pests. It may be a solution of difethialone in a solvent for difethialone, with the exclusion of a racemic mixture of dextrorotatory and laevorotatory enantiomers of said homo-stereoisomer of difethialone. It may also be a solution of difethialone in a solvent for difethialone and in which said homo-stereoisomer of difethialone is predominantly in dextrorotatory enantiomer form. It may also be a solution of difethialone in a solvent for difethialone and in which the difethialone is predominantly in the dextrorotatory enantiomer form of said homo-stereoisomer of difethialone. It may also be a suspension or an emulsion of difethialone in a liquid medium.

The invention thus also relates to a rodenticidal bait in which the difethialone is optically active. However, it is not excluded for the difethialone of the rodenticidal bait according to the invention to be optically inactive.

Advantageously, the rodenticidal bait comprises at least one dye. Such a dye makes it possible in particular to give said rodenticidal bait a colour that is readily detectable and identifiable by a person handling the rodenticidal bait.

Advantageously, the rodenticidal bait comprises at least one preserving agent capable of ensuring its conservation during its storage. Advantageously, the rodenticidal bait comprises at least one bittering compound such as denatonium benzoate, also known as Bitrex®, which is intended to reduce the risks of accidental consumption by non-target organisms.

Advantageously, in one particular variant, the composition and the rodenticidal bait according to the invention exclusively comprise difethialone—in which the homo-stereoisomer is not a racemic mixture—as rodenticidal substance. In particular, the composition and the rodenticidal bait according to the invention are free of any other anticoagulant substance for rodenticidal use. However, in this variant according to the invention, the composition and the rodenticidal bait may comprise any pest-control substance other than a rodenticide, such as an insecticidal and/or acaricidal substance.

Advantageously, in another particular variant, the composition and the rodenticidal bait according to the invention comprise difethialone with the exclusion of a racemic mixture of laevorotatory and dextrorotatory enantiomers of said homo-stereoisomer and at least one other substance different from difethialone as rodenticidal substance. This other rodenticidal substance different from difethialone may be another anticoagulant substance—especially of the anti-vitamin K type or not—or another non-anticoagulant rodenticidal substance.

The invention also relates to a process for controlling target rodent pests, in which there is spread an amount of rodenticidal bait comprising:

-   -   at least one excipient that is edible for target rodent pests;         and     -   the dextrorotatory enantiomer of the configurational         stereoisomer of difethialone, named homo-stereoisomer, the         formula of which is         3-(4′-bromobiphenyl-4-yl)-1-(4-hydroxythiocoumarin-3-yl)-1,2,3,4-tetrahydronaphthalene,         in which carbons 1 and 3 of the 1,2,3,4-tetrahydronaphthalene         group of said homo-stereoisomer have the same absolute         configuration;         with the exclusion of a racemic mixture of the dextrorotatory         and laevorotatory enantiomers of said homo-stereoisomer of         difethialone.

The invention also relates to a process for controlling target rodent pests, in which there is spread an amount of rodenticidal bait according to the invention, said amount of bait being sufficient to be rodenticidal.

An amount of rodenticidal bait comprising said homo-stereoisomer of difethialone predominantly in dextrorotatory enantiomer form, which has reduced hepatic persistence in the target rodent pest and, surprisingly, rodenticidal efficacy that is at least maintained, is thus spread. The process according to the invention thus makes it possible to limit the secondary poisoning of non-rodent mammals and birds that are liable to feed on poisoned rodents which are dead or alive but comprising a reduced amount—and especially a non-lethal amount—of difethialone. The process according to the invention also makes it possible to limit such secondary poisoning of non-rodent mammals and birds that are liable to preferentially consume the viscera—in particular the liver—of said dead or live poisoned rodents.

Advantageously and according to the invention, said homo-stereoisomer of difethialone is predominantly in dextrorotatory enantiomer form. Advantageously and according to the invention, the difethialone is predominantly in the dextrorotatory enantiomer form of said homo-stereoisomer of difethialone.

Advantageously and as a variant according to the invention, the following are chosen in combination:

-   -   the edible excipient;     -   a proportion of dextrorotatory enantiomer of said         homo-stereoisomer of difethialone relative to the         homo-stereoisomer of difethialone;     -   a proportion of dextrorotatory enantiomer of said         homo-stereoisomer of difethialone relative to the difethialone;     -   a mass proportion of difethialone relative to the rodenticidal         bait; and     -   an amount of spread bait;         so that target rodent pests consume an amount of difethialone         that is sufficient to be lethal to said target rodent pests         which consume said bait in the course of a single period of 24         consecutive hours.

A rodenticidal bait according to this variant of the invention is a bait that is mortal in a single intake, or a “one-shot” bait. Advantageously and according to this variant of the invention, the mass proportion of difethialone in the rodenticidal bait is between 2 ppm and 200 ppm, especially between 5 ppm and 100 ppm, preferably between 10 ppm and 50 ppm, more preferentially between 15 ppm and 50 ppm.

Advantageously and in another variant according to the invention, the following are chosen in combination:

-   -   the edible excipient;     -   a proportion of dextrorotatory enantiomer of said         homo-stereoisomer of difethialone relative to the         homo-stereoisomer of difethialone;     -   a mass proportion of difethialone relative to the rodenticidal         bait; and     -   an amount of spread bait;         so that target rodent pests consume an amount of difethialone:     -   which is non-lethal to target rodent pests, i.e. which is         generally non-lethal to target rodent pests which consume said         bait over a period of 24 consecutive hours; and     -   which is sufficient to be lethal to target rodent pests which         consume said bait over several 24-hour periods.

Advantageously, the 24-hour periods are consecutive.

This other variant of the invention is thus also directed towards a process for controlling target rodent pests, in which there is spread an amount of rodenticidal bait that is lethal to target rodent pests durably consuming this rodenticidal bait and non-lethal to non-target rodents or animals accidentally consuming this rodenticidal bait. This is then referred to as a “multi-dose” or “multi-feeding” control process. In such a process according to the invention, the consumption of rodenticidal bait by a target rodent pest over a period of 24 hours is generally insufficient to result in the death of said rodent, whereas repeated consumption of rodenticidal bait over at least two days results in the death of the target rodent pest.

The invention is thus directed towards a process for controlling a population of target rodent pests, in which target rodent pests are provided with an amount of rodenticidal bait that is liable to be ingested by the target rodent pests, said amount of rodenticidal bait being sufficient to kill target rodent pests which consume said rodenticidal bait over several days.

Advantageously, in this other variant of the invention, an amount of rodenticidal bait is spread so that target rodent pests consume an amount of difethialone that is sufficient to be lethal to said target rodent pests which consume said bait over several 24-hour periods, said periods being consecutive.

Advantageously and in this other variant according to the invention, the mass proportion of difethialone is between 5 ppm and 50 ppm, especially between 5 ppm and 30 ppm—especially about 15 ppm—relative to the rodenticidal bait.

Advantageously, the amount of rodenticidal bait spread, the mass proportion of difethialone relative to the rodenticidal bait and the proportion of dextrorotatory enantiomer of said homo-stereoisomer of difethialone relative to the difethialone are adapted so that the consumption of the rodenticidal bait is lethal to target rodent pests which daily consume bait over at least two 24-hour periods, especially from 3 to 7 periods, at least two—and especially all—of the periods being consecutive.

Advantageously, in this other variant of a process according to the invention, since the proportion of dextrorotatory enantiomer of said homo-stereoisomer of difethialone is greater than 95%—especially about 100%—relative to the difethialone, the mass proportion of difethialone relative to the rodenticidal bait is between 5 ppm and 50 ppm, especially about 15 ppm.

In a process according to the invention, an amount of rodenticidal bait that is sufficient to satisfy their daily appetite is provided to target rodent pests, said rodenticidal bait comprising a major proportion of dextrorotatory enantiomer of said homo-stereoisomer of difethialone.

In a process according to the invention, the amount of rodenticidal bait spread, the proportion of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone relative to the difethialone and the mass proportion of difethialone relative to the rodenticidal bait are adapted so as to allow consumption of rodenticidal bait for several days by target rodent pests, while at the same time limiting:

-   -   the risks of primary intoxication of non-target mammals and         birds which are liable to consume such a rodenticidal bait only         occasionally and accidentally;     -   the risks of secondary intoxication, for example of predators of         target rodents, which are liable to consume target rodents—dead         or live—that have ingested an amount of said bait.

Advantageously and according to the invention, the amount of bait spread, the mass proportion of difethialone relative to the bait and the proportion of dextrorotatory enantiomer of said homo-stereoisomer of difethialone are adapted to limit the (total) amount of difethialone in the liver of rodents which have died by consuming said bait.

The invention also relates to a chromatographic process for obtaining a dextrorotatory enantiomer of a configurational stereoisomer of difethialone, named homo-stereoisomer, the formula of which is 3-(4′-bromobiphenyl-4-yl)-1-(4-hydroxythiocoumarin-3-yl)-1,2,3,4-tetrahydronaphthalene, in which carbons 1 and 3 of the 1,2,3,4-tetrahydronaphthalene group of said homo-stereoisomer have the same absolute configuration, in which process:

-   -   a high-pressure liquid chromatography column of dimensions 150×2         mm, and comprising a chiral stationary phase constituted of         cellulose tris(4-methylbenzoate) particles, said particles         having a mean size of 3 μm and having a mean pore size of 1000         Å, is chosen;     -   a mixture formed from acetonitrile (A) and water comprising 0.1%         by volume of formic acid (B), with an A/B volume ratio of 80/20         and with a flow rate of the liquid mobile phase in the         chromatography column of 0.25 mL/minute, is chosen as liquid         mobile phase;     -   separation of the configurational stereoisomers of difethialone         is performed at room temperature, during which:     -   a liquid composition comprising said dextrorotatory enantiomer         of said homo-stereoisomer of difethialone is introduced into the         top of the chromatography column; and then     -   the liquid composition is entrained with the mobile phase in the         chromatography column under conditions suitable for separating         the configurational stereoisomers of difethialone; and     -   a fraction of the mobile phase comprising said dextrorotatory         enantiomer of said homo-stereoisomer of difethialone is         collected with a retention time t₄ having a value such that         t₁<t₂<t₃<t₄; t₁, t₂ and t₃ representing the retention times of         each of the configurational stereoisomers of difethialone         different from the dextrorotatory enantiomer of said         homo-stereoisomer of difethialone, separately from a         laevorotatory enantiomer of said homo-stereoisomer of         difethialone with a retention time t₁ and separately from the         laevorotatory and dextrorotatory enantiomers of a         configurational stereoisomer of difethialone, named         hetero-stereoisomer, in which carbons 1 and 3 of the         1,2,3,4-tetrahydronaphthalene group of said hetero-stereoisomer         have different absolute configurations, and of retention times         t₂ and t₃; and then     -   the liquid mobile phase of said fraction is removed so as to         obtain said dextrorotatory enantiomer of said homo-stereoisomer         of difethialone.

The invention also relates to said dextrorotatory enantiomer of said homo-stereoisomer of difethialone obtained via a process according to the invention.

The invention also relates to a configurational stereoisomer of difethialone, to a process for obtaining such a configurational stereoisomer, to a composition comprising such a configurational stereoisomer, to a rodenticidal bait and to a process for controlling target rodent pests, characterized in combination by all or some of the characteristics mentioned hereinabove or hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aims, characteristics and advantages of the invention will emerge on reading the following description and the examples, which are given for purely non-limiting purposes and which refer to the attached figures, in which:

FIG. 1 is a chromatogram of an analysis by high-pressure liquid chromatography on a chiral column of difethialone (top) and of the dextrorotatory enantiomer of said purified homo-stereoisomer of difethialone (bottom);

FIG. 2 is a proton NMR spectrum at 300 MHz of said homo-stereoisomer of difethialone;

FIG. 3 is a proton NMR spectrum at 300 MHz of said hetero-stereoisomer of difethialone;

FIG. 4 is a proton NMR spectrum at 500 MHz of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone according to the invention;

FIG. 5 is a ¹³C carbon NMR spectrum at 500 MHz of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone;

FIG. 6 is an analysis by proton NMR (¹H-NMR) correlation spectroscopy at 500 MHz of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone;

FIG. 7 is a circular dichroism spectrum of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone;

FIG. 8 is a representation in graph form of the change over time of the hepatic concentration in rats (male and female) of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone (●) and of the laevorotatory enantiomer of said homo-stereoisomer of difethialone (◯);

FIG. 9 is a representation in graph form of the change over time of the hepatic concentration in rats (male and female) of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone (●), of said homo-stereoisomer of difethialone (□) and of said hetero-stereoisomer of difethialone (▪); and

FIG. 10 is a representation in graph form of the change over time of the mean hepatic concentration in rats of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone (●) and of the total difethialone (Δ).

DETAILED DESCRIPTION OF THE INVENTION

A. Purification of the Dextrorotatory Enantiomer of said Isolated Homo-Stereoisomer of Difethialone

A.1. Identification of Said Homo-Stereoisomer of Difethialone and of said Hetero-Stereoisomer of Difethialone

The homo-stereoisomer of difethialone is identified by proton magnetic resonance (¹H-NMR) spectroscopy. The homo-stereoisomer of difethialone dissolved in CDCl₃ has a multiplet with a chemical shift (δ) of between 4.9 ppm and 5.1 ppm and corresponding to the proton borne by carbon 1 of the 1,2,3,4-tetrahydronaphthalene group of difethialone as illustrated in FIG. 2.

It is possible to distinguish the hetero-stereoisomer of difethialone and the homo-stereoisomer of difethialone by their proton NMR spectra. In the proton NMR spectrum acquired in CDCl₃, the chemical shift of the proton borne by carbon 1 of the 1,2,3,4-tetrahydronaphthalene group of said hetero-stereoisomer of difethialone (FIG. 3) is between 5.2 ppm and 5.4 ppm.

A.2. Separation of the Laevorotatory and Dextrorotatory Enantiomers of said Homo-Stereoisomer of Difethialone by High-Pressure Liquid Chromatography on a Chiral Column

The inventors solved the complex and hitherto unresolved problem of separating the configurational isomers of difethialone and in particular the laevorotatory and dextrorotatory enantiomers of said homo-stereoisomer of difethialone. They succeeded in performing separation of the enantiomers of said homo-stereoisomer of difethialone by high-pressure (high-performance) liquid chromatography on a LUX® Cellulose-3 column (Phenomenex, Le Pecq, France) of dimensions 150×2 mm and comprising a chiral stationary phase constituted of porous particles of cellulose tris(4-methylbenzoate), with a particle size of 3 μm and a porosity of 1000 Å and using, as mobile phase, a mixture formed from acetonitrile (A) and water comprising formic acid in a volume proportion of 0.1% (B), the AB volume proportion being 80/20. The flow rate of the mobile phase in the column is 0.25 mL/minute and the separation is performed at a temperature of 23.2° C. The solution containing the sample to be analysed is at a concentration of 1 μg of difethialone per millilitre in acetonitrile and is filtered through a regenerated cellulose membrane with a cut-off threshold of 0.2 μm. The volume of solution containing the sample to be analysed injected onto the column is 1 μL.

In a process for separating the enantiomers of said homo-stereoisomer of difethialone, it is possible to detect said enantiomers leaving the high-pressure liquid chromatography column by tandem mass spectrometry (MS/MS) in negative electrospray ionization mode (ESI: ElectroSpray Ionization). The temperature of the nebulizer gas is 350° C. and its flow rate is 8 L/minute. The pressure of the nebulizer gas is brought to 2700 hPa. In particular, the MRM (“Multiple Reaction Monitoring”) transitions m/z 537.1→151.0 and m/z 537.1→78.9, corresponding to the difethialone signals, are detected. FIG. 1 represents the chromatograms of difethialone (top) and of the dextrorotatory enantiomer of the isolated homo-stereoisomer of difethialone (bottom).

Under these experimental conditions, the value of the retention time (t₄) for said dextrorotatory enantiomer of said homo-stereoisomer according to the invention is about 14.4 minutes as described in FIG. 1. By way of comparison, the value of the retention time (t₁) for the laevorotatory enantiomer of said homo-stereoisomer is about 8.1 minutes as described in FIG. 1, and as such the dextrorotatory and laevorotatory enantiomers of said homo-stereoisomer may be efficiently separated by high-pressure liquid chromatography on a chiral column.

The value of the retention time (t₃) for the dextrorotatory enantiomer of said hetero-stereoisomer of difethialone is about 11.7 minutes and the value of the retention time (t₂) for the laevorotatory enantiomer of said hetero-stereoisomer of difethialone is about 9.4 minutes. Thus, under these experimental conditions, the order of elution of the configurational stereoisomers of difethialone is such that t₁<t₂<t₃<t₄.

It is possible under these experimental conditions (stationary phase, mobile phase, temperature) to perform a preparative separation of the laevorotatory and dextrorotatory enantiomers of said homo-stereoisomer of difethialone by using a similar stationary phase with a particle size of greater than 3 μm, and a chromatography column of larger dimensions, especially a diameter of 20 mm.

B. Structural Characterization of the Dextrorotatory Enantiomer of said Homo-Stereoisomer of Difethialone

B.1. UV Spectroscopy

The UV spectrum of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone dissolved in chloroform (CHCl₃) shows absorbance peaks centred at 238.2 nm and 259.5 nm.

B.2. Optical Rotation

The inventors characterized the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in isolated form by means of its optical rotation (also known as the optical activity or circular birefringence), i.e. its ability to deviate the polarization plane of polarized light. Deviation of the polarization plane of polarized light clockwise facing the polarized light beam characterizes a dextrorotatory solution, and deviation of the polarization plane of polarized light anticlockwise facing the polarized light beam characterizes a laevorotatory solution and compound.

The optical rotation of a solution of dextrorotatory enantiomer of said homo-stereoisomer of difethialone in chloroform (CHCl₃) is measured at a concentration of 11.6 g/L. The optical rotation of this solution is measured by means of a P 2000 digital polarimeter (JASCO, Bouguenais, France) operating with excitatory light with a wavelength of 589 nm. The mean optical rotation α obtained on two series of ten measurements is 2.308°. The specific optical rotation at 25° C. [α]^(25° C.) _(589nm) for the dextrorotatory enantiomer of said homo-stereoisomer of difethialone dissolved in chloroform, measured on the sodium D line (589 nm), is +19.9°.

B.3. Circular Dichroism

The circular dichroism spectrum of the isolated dextrorotatory enantiomer of said homo-stereoisomer of difethialone reflects the difference in absorbance (ΔA=A_(L)−A_(R)) of the two waves of left circular polarization (LCP) of intensity A_(L) and of right circular polarization (RCP) of intensity A_(R). This makes it possible to distinguish the dextrorotatory and laevorotatory enantiomers of said homo-stereoisomer of difethialone. This difference in absorbance of the two circularly polarized waves is measured in a J-815 circular dichroism spectrometer (JASCO, Bouguenais, France). 2 mL of a solution of dextrorotatory enantiomer of said homo-stereoisomer of difethialone in methanol (CH₃OH) at a concentration of 0.96 mg/mL are prepared. The solution is transferred into a quartz spectrophotometer cuvette. The circular dichroism spectrum of the solution is measured at 25° C. between 210 nm and 400 nm. The circular dichroism spectrum of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone measured under these conditions is shown in FIG. 7. The circular dichroism value is positive between the wavelengths of between 220 nm and 300 nm.

B.4. Nuclear Magnetic Resonance

FIGS. 2 and 3 represent, respectively, a proton nuclear magnetic resonance spectrum (¹H-NMR) at 300 MHz of the homo-stereoisomer of difethialone in CDCl₃ (FIG. 2) and a proton nuclear magnetic resonance spectrum at 300 MHz in CDCl₃ of the hetero-stereoisomer of difethialone in CDCl₃ (FIG. 3). Said homo-stereoisomer of difethialone has (FIG. 2) a multiplet whose chemical shift (δ) is between 4.9 ppm and 5.1 ppm, corresponding to carbon 1 of the 1,2,3,4-tetrahydronaphthalene group of said homo-stereoisomer of difethialone. Said hetero-stereoisomer of difethialone has (FIG. 3) a multiplet whose chemical shift (δ) is between 5.2 ppm and 5.4 ppm.

FIG. 4 is a proton NMR spectrum at 500 MHz of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in CDCl₃. The proton NMR spectra of the dextrorotatory and laevorotatory enantiomers of said homo-stereoisomer of difethialone are indistinguishable from each other.

FIG. 6 is a two-dimensional proton NMR (2D ¹H-NMR) spectrum obtained by correlation spectroscopy of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone dissolved in CDCl₃ at a concentration of 40 mg/mL acquired on a Brüker Avance III HD spectrometer (500 MHz) equipped with a Prodigy motorized multi-core direct cryoprobe. It allows identification of the coupling of the proton borne by carbon 1 of the 1,2,3,4-tetrahydronaphthalene group of said homo-stereoisomer of difethialone (5.1 ppm) with the protons borne by carbon 2 of the 1,2,3,4-tetrahydronaphthalene group at 2.4 ppm.

FIG. 5 is a ¹³C NMR spectrum of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone dissolved in CDCl₃ at a concentration of 40 mg/mL acquired on a Brüker Avance III HD spectrometer (500 MHz) equipped with a Prodigy motorized multi-core direct cryoprobe. It allows identification of the 31 carbon atoms of difethialone. The ¹³C-NMR spectrum of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone is not distinguished from the ¹³C-NMR spectrum of the laevorotatory enantiomer of said homo-stereoisomer of difethialone. However, said homo-stereoisomer of difethialone has a characteristic signal between 34 ppm and 38 ppm which is distinctive for said hetero-stereoisomer of difethialone.

C. Extraction of Difethialone from the Liver of Rats Treated with Difethialone for the Purpose of Analysis of the Configurational Stereoisomers of Difethialone

C.1. Homogenization of the Liver Sample

About 0.525 g (±0.025 g) of rat liver is weighed out accurately and placed in a 50 mL polypropylene tube. 10 mL of acetone are added and the suspension is homogenized using an Ultra-Turrax® homogenizer/disperser for a time of about 30 seconds. The homogenizer/disperser shaft is rinsed with hot water and then twice with 20 mL of acetone in a polypropylene tube. The homogenate is centrifuged for 5 minutes at a centrifugation speed of 3000 rpm (revolutions per minute). The supernatant is collected and transferred into a test tube. The sample is subjected to evaporation under a stream of nitrogen (N₂) at a temperature of 40° C. so as to form a dry extract.

C.2. Lipid Removal

1 mL of acetonitrile is added to the tube containing the dry extract so as to dissolve it. The acetonitrile solution is washed twice successively with 1 mL of hexane. The lipid-free extract is dried under a stream of nitrogen (N₂) at a temperature of 40° C. and is then taken up in 0.5 mL of methanol and dissolved by vortex stirring. 0.5 mL of ultra-pure (Milli-Q) water is then added. The sample is vortex-homogenized.

C.3. Solid-Phase Extraction (SPE) of Difethialone

1 mL of dichloromethane (CH₂Cl₂), then 1 mL of methanol (CH₃OH), then 1 mL of ultra-pure (Milli-Q) water are passed through an Oasis HLB 1 cc cartridge (WAT094225, Waters). The lipid-free liver extract (1 mL MeOH/Milli-Q H₂O) containing difethialone is then loaded onto the top of the preconditioned cartridge. The liver extract penetrates through the cartridge by gravity on contact with the solid phase of the cartridge. 1 mL of washing solution formed from methanol and ultra-pure water in a 90/10 volume proportion is then loaded onto the top of the cartridge. The cartridge is dried by suction under vacuum connected to the bottom of the cartridge. 1 mL of eluting solution formed from dichloromethane and methanol in a 90/10 volume proportion is then loaded onto the top of the cartridge and an eluate comprising difethialone is collected at the bottom of the cartridge. The solvent of the eluate is evaporated off under a stream of nitrogen (N₂) at a temperature of 40° C. The sample is taken up in 0.5 mL of acetonitrile and the acetonitrile solution containing difethialone is filtered through a 0.2 μm filter.

C.4. Analysis

The acetonitrile solution containing difethialone is analysed by high-pressure liquid chromatography on a LUX® Cellulose-3 chiral column (Phenomenex, Le Pecq, France) (150×2 mm, particle size of 3 μm) as described in point A2) above.

D. Study of the Hepatic Persistence of the Configurational Stereoisomers of Difethialone in Rats

A solution of a mixture of homo-stereoisomer (DFN-Homo-SI) and of hetero-stereoisomer (DFN-Hetero-SI) of difethialone in a mixture of vegetable oil and 5% DMSO is administered by tube-feeding (“per os”) to 8-week-old coumaphen-sensitive rats (male and female Rattus norvegicus) weighing about 200 g. The molar proportion of homo-stereoisomer is 40% and the molar proportion of hetero-stereoisomer is 60%. Each configurational stereoisomer of difethialone is formed from a racemic mixture of the two enantiomers of said corresponding configurational stereoisomer.

On D0, a tube-feeding solution is administered to the rats so that the amount of difethialone ingested by each rat is about 3.4 mg per kilogram of rat. To avoid haemorrhage, the tube-fed rats are treated daily by subcutaneous administration of a dose of vitamin K1 (as haemorrhage antidote) at a rate of 0.1 U per 200 g of live rat weight.

At 4 hours (H+4), 9 hours (H+9), 24 hours (H+24), 120 hours (H+120), 168 hours (H+168) and 216 hours (H+216) after tube-feeding, three male rats and three female rats anaesthetized beforehand with isoflurane are euthanized, the liver of the euthanized rats is removed, the difethialone is then extracted from the liver and the amount of each of the configurational stereoisomers of difethialone is assayed, the area under the peaks in the chromatogram obtained is measured and each configurational stereoisomer is quantified by comparison with a calibration curve. The following are assayed:

-   -   the dextrorotatory enantiomer of said homo-stereoisomer;     -   the laevorotatory enantiomer of said homo-stereoisomer;     -   the dextrorotatory enantiomer of said hetero-stereoisomer;     -   the laevorotatory enantiomer of said hetero-stereoisomer;         present in the liver of the tube-fed rats.

The results are shown in FIGS. 8, 9 and 10, in which the content of said dextrorotatory enantiomer of said homo-stereoisomer of difethialone expressed in nanograms of enantiomer per gram of liver (ng/g) is given as a function of the time (in hours) after tube-feeding of the rats.

In FIG. 8, the content of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in the liver (mean of the contents measured on six rats expressed in nanograms of said enantiomer per gram of liver (ng/g)) is represented by filled circles (●) and the content of the laevorotatory enantiomer of said homo-stereoisomer of difethialone in the liver (mean of the contents measured on six rats expressed in nanograms of said enantiomer per gram of liver (ng/g)) is represented by empty circles (◯). The hepatic content of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone decreases more rapidly than the hepatic content of the laevorotatory enantiomer of said homo-stereoisomer of difethialone, whereas the initial proportion of the two enantiomers ingested by the rats is identical (20%). The values for the measured hepatic contents are given in table 1 below, in which “DFN-Homo-SI” represents said homo-stereoisomer of difethialone, “DFN-Homo-dextro” represents the dextrorotatory enantiomer of the homo-stereoisomer of difethialone according to the invention, “DFN-Homo-laevo” represents the laevorotatory enantiomer of the homo-stereoisomer of difethialone, “DFN-Hetero-SI” represents said hetero-stereoisomer of difethialone, “DFN-Hetero-laevo” represents the laevorotatory enantiomer of the hetero-stereoisomer of difethialone and “DFN-Hetero-dextro” represents the dextrorotatory enantiomer of the hetero-stereoisomer of difethialone.

TABLE 1 Hepatic content, ng/g Time Total difethialone after DFN-Homo-SI DFN-Hetero-SI tube-feeding, DFN-Homo- DFN-Homo- DFN-Hetero- DFN-Hetero- hours dextro laevo dextro laevo 4 4566 5692.5 10589.5 5380.5 9 4692.5 7141 12155.5 5869.5 24 1243.5 4403.5 8102 3613.5 48 720.5 3874 7974.5 2804 120 192.5 2087 5431.5 1211 168 129 878.5 3011 392.5 216 77.5 1224 4030.5 545

In FIG. 9, the relative content of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in the liver (mean of the contents measured on six rats) expressed in arbitrary units for comparative purposes is represented by filled circles (●), the relative content of said homo-stereoisomer is represented by empty squares (

) and the relative content of said hetero-stereoisomer is represented by filled squares (▪). The hepatic content of the dextrorotatory enantiomer of the homo-stereoisomer of difethialone according to the invention decreases more rapidly than the hepatic content of said homo-stereoisomer of difethialone and than the hepatic content of said hetero-stereoisomer of difethialone. In addition, the hepatic content of said homo-stereoisomer of difethialone decreases more rapidly than the hepatic content of said hetero-stereoisomer of difethialone. The representation in FIG. 9 takes into account the fact that the dextrorotatory enantiomer of the homo-stereoisomer of difethialone accounts for 20% of the difethialone ingested by the rats and that each of said homo-stereoisomer of difethialone (DFN-Homo-SI) and of said hetero-stereoisomer of difethialone (DFN-Hetero-SI) accounts for 40% and 60%, respectively, of the difethialone ingested by the rats. The hepatic content values are related to the percentage of each configurational stereoisomer in the composition ingested by the rats.

In FIG. 10, the change over time of the content of dextrorotatory enantiomer of said homo-stereoisomer of difethialone in the liver (mean of the contents measured on six rats expressed in nanograms of enantiomer per gram of liver (ng/g)) is represented (left-hand scale) by filled circles (●) and the change over time of the total difethialone content is represented (right-hand scale) by empty triangles (Δ). The hepatic content of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone according to the invention decreases more rapidly than the hepatic content of the total difethialone. The representation in FIG. 10 takes into account the fact that the dextrorotatory enantiomer of the homo-stereoisomer of difethialone accounts for 20% of the difethialone ingested by the rats and the total difethialone accounts for 100% of the difethialone ingested by the rats.

The dextrorotatory enantiomer of the homo-stereoisomer of difethialone shows significantly lower hepatic persistence relative to all the other configurational stereoisomers of difethialone in the rats.

E. Rodenticidal Bait Comprising a Mass Proportion of 14.7 ppm of Difethialone

A pasty rodenticidal bait according to the invention is prepared by dispersing an amount of difethialone in an edible excipient comprising vegetable fat and cereal flour. The measured proportion of difethialone relative to the bait is 14.7 ppm (14.65 mg of dextrorotatory enantiomer of said homo-stereoisomer of difethialone per kilogram of bait) and the proportion of dextrorotatory enantiomer of said homo-stereoisomer relative to the difethialone is 99.7%. The bait also comprises a mass proportion of 0.3% of laevorotatory enantiomer of said hetero-stereoisomer of difethialone relative to the difethialone.

On D0, ten coumaphen-sensitive Sprague-Dawley rats (five male and five female SD rats) are placed in individual cages with a rodenticide-free reference feed. On D3, each rat is weighed, and 50 g of rodenticidal bait as described above are then provided to each rat. This provision of 50 g of rodenticidal bait is renewed daily. The bait consumed by the rats is made up to 50 g of bait on D4, D5 and D6. Starting from D7, the residual rodenticidal baits are removed and rodenticide-free feed is provided to all the rats. The rats are monitored for 3 weeks.

The mean amounts of bait consumed daily by a rat at D4, D5, D6 and D7 expressed in grams per day are given in table 2 below.

TABLE 2 Bait consumed, g Mean Standard deviation D4 17.8 5.7 D5 16.0 4.7 D6 14.2 4.1 D7 9.4 3.8

It should be noted that no rat consumed a daily amount of bait of less than 1 g/day. All the rats (100%) die between D9 and D10. The mortality is 100% on D10. The mean bait consumption, expressed as grams of bait per kilogram of rat, is 50.5 g of bait.

The hepatic content of the dextrorotatory enantiomer of the homo-stereoisomer of difethialone (“DFN-Homo-dextro”) according to the invention for each of the rats which die between D9 and D10 is measured by high-pressure liquid chromatography analysis on a chiral column. The values, expressed as micrograms of “DFN-Homo-dextro” and difethialone per gram of liver, are given in table 3 below. The mean amount of “DFN-Homo-dextro” enantiomer retained in the liver of each rodent (male and female) on their death corresponds to 4.7% of the amount of difethialone ingested by each rodent.

TABLE 3 Hepatic content, μg/g DFN-Homo-dextro Total difethialone Mean 6.1 6.1 Standard deviation 4.5 4.5

For comparative purposes, the difethialone content retained in the liver of rats which consumed a bait comprising 15 ppm of difethialone of the prior art is about 110 μg/gram.

The bait containing a 14.7 ppm dose of difethialone makes it possible to obtain a mortality rate of 100% while minimizing the risks of secondary intoxication of animals—especially birds—which prey or carrion-feed on weakened target rodent pests that have consumed a rodenticidal bait.

It goes without saying that the invention may be the subject of numerous implementation variants and applications. In particular, a composition, a rodenticidal bait and a process for controlling target rodent pests are subject to an infinite number of variants both in the formulation of the bait and in the embodiments of the process. 

The invention claimed is:
 1. Rodenticidal bait comprising at least one excipient that is edible for target rodent pests and a rodenticidal amount of difethialone in the form of a dextrorotatory enantiomer of a configurational stereoisomer of difethialone, named homo-stereoisomer, the formula of which is 3-(4′-bromobiphenyl-4-yl)-1-(4-hydroxythiocoumarin-3-yl)-1,2,3,4-tetrahydronaphthalene, in which carbons 1 and 3 of the 1,2,3,4-tetrahydronaphthalene group of said homo-stereoisomer have the same absolute configuration, with the exclusion of a racemic mixture of the dextrorotatory and laevorotatory enantiomers of said homo-stereoisomer of difethialone, and the rodenticidal bait comprising an amount of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone such that the ratio of this amount to the amount of difethialone in the rodenticidal bait is greater than 90%.
 2. Bait according to claim 1, further comprising an amount of the dextrorotatory enantiomer of said homo-stereoisomer of difethialone in the rodenticidal bait such that the ratio of this amount to the amount of difethialone in the rodenticidal bait is greater than 97%.
 3. Bait according to claim 1, wherein the edible excipient comprises at least one food chosen from the group formed from cereal seeds, cereal seed meals, cereal seed flours, cereal seed flakes, cereal bran and non-cereal seeds.
 4. Bait according to claim 1 further comprising a mass amount of difethialone such that the ratio of this mass amount of difethialone to the mass amount of rodenticidal bait is less than 200 ppm.
 5. Bait according to claim 1, wherein said dextrorotatory enantiomer of said homo-stereoisomer of difethialone is the enantiomer of said homo-stereoisomer of difethialone which is less persistently retained in the liver of target rodent pests. 